Thinly slice the red onion and place it in a small bowl. Squeeze the lemon juice over the onions and stir to combine. Let the onions sit for at least 5 minutes while you prepare the other ingredients—the acid will soften the onions and mellow their sharp bite, creating a more balanced flavor throughout the salad. I find this quick pickling step makes a huge difference in how well the salad tastes, especially if you're making it a few hours ahead.

In a large mixing bowl, combine the drained tuna, rinsed cannellini beans, and the chopped fresh parsley. Gently fold these ingredients together, being careful not to break up the tuna too much—you want some texture and flakes throughout the salad, not a mushy consistency. This creates your flavor foundation before adding the acidic and spiced components.

Pour the pickled red onions with all their lemon juice into the tuna and bean mixture from Step 2. Add the capers, red chile flakes, black pepper, and 2 tablespoons of olive oil. Toss everything together gently but thoroughly to distribute the flavors evenly. I like to taste as I go here—the chile flakes and capers can be quite punchy, so you may want to adjust them to your preference before adding salt.

Taste the salad and add salt to your preference, keeping in mind that the capers and tuna already contribute some saltiness. If the salad needs more brightness, squeeze a bit more fresh lemon juice over it. If it seems dry or you want to enhance the olive oil flavor, drizzle in a touch more extra virgin olive oil. Let the salad rest for at least 10 minutes before serving so all the flavors meld together beautifully.